1. Field of the Invention
The present invention relates to a method and circuit arrangement for offset correction of a measurement bridge, especially in connection with a sensor arrangement, in which a measured signal of a full bridge branch and at least one measured signal of at least one half bridge branch are evaluated.
2. Description of the Related Art
A sensor evaluation circuit with a measurement bridge, as is described for example from DE 197 28 381 A1, generally permits a highly precise and reliable measurement of very small signals in the μV. Especially when the measurement bridge is used in safety-critical systems, such as an anti-locking system (ABS) or an electronic stability program in motor vehicles, frequently the same measurement variables and their results from two sensors are compared in a control unit. Alternatively self-monitoring is required in order to be able to reliably detect a faulty sensor and to prevent erroneous operation of the system.
In order to provide a sensor with self-monitoring capability, for example a sensor of the above-described type, in which the measured signal is obtained by analysis of a Wheatstone bridge circuit, besides analysis of the full bridge signal path one of the two half bridge signal paths are also analyzed. This sort of method is used with piezo-resistively operating pressure sensors based on polysilicon.
However the monitoring signal obtained in the above-described manner according to the state of the art disadvantageously has half signal amplitude and a not inherently compensated offset, which exceeds the actual signal amplitude by about an order of magnitude. The results can be a significantly reduced robustness of the monitoring path and thus a significantly increased failure risk for the entire sensor because of the integrated monitoring. The signals from the full bridge and half bridge are specifically corrected in the usual manner in an evaluation circuit and amplified.
In a calibration cycle during manufacturing of the sensor according to the state of the art the installed sensor is adjusted by means of operating parameters and the correction parameters are stored in a non-volatile memory in an analysis circuit. The full bridge and half bridge signals are adjusted according to a correction and an alarm is activated when there is a significant variation. However the conductivity of the sensor is limited by the monitoring path so that frequently sensors, which output the correct useful signal, however signal an error or fault because of a drift in the half bridge path. A clear increase of robustness of the monitoring path should then be achievable.